THE ECOLOGY OF SASKATCHEWAN SPHAERIIDAE (MOLLUSCA; BIVALVIA): AN EVALUATION OF SOME COMPONENTS OF THEIR ENVIRONMENT
Murray, Alan Roderick
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The importance of some physical and chemical aspects of the environment of sphaeriids was assessed experimentally and analytically. The experimental phase measured the resistance of two species to lethal levels of high temperature, low dissolved oxygen, and different lake waters. The analytical phase evaluated the relationships between 'the distribution and abundance of 13 species and 17 environmental variables derived from measurements of depth, maximum water temperature, minimum dissolved oxygen, mineral content of the water, and morphometry in 27 Saskatchewan lakes. Species tested experimentally were Pisidium aonventus Clessin, collected from 19 m in an oligotrophic lakeon the Precambrian Shield in northern Saskatchewan, and P. lilljeborgi Clessin, collected from 1 m in a small, shallow, prairie slough in central Saskatchewan. Collections of each species were maintained in the laboratory in water of the lakes from which they had been obtained. Specimens of each species were acclimated to 4 C and 18 C prior to testing. In one series of experiments, each species was tested in water from each lake and exposed to three lethal high temperatures at 0.1 mg °2/1 and at saturation. P. aonventus was tested at 28, 30 and 32 C; P. lilljeborgi was tested at 32, 33.5, and 35 C. In a second series of experiments, each species was tested in water from each lake at 32 C and at 0.1 mg °2/1,1.0 mg °2/1, and saturation. Median lethal times or LT50's, derived by probit analysis, were assessed by factorial analysis of variance. LT50's for specimens of P. lilljeborgiwere 5-10 times longer than those for P. aonventus at 32 C. LT50's for specimens of P. aonventus and P. lilljeborgiwere significantly increased by the higher acclimation temperature and significantly decreased by exposure to low oxygen but the effects were much greater for P. aonventus. LTSO's for specimens of P. conventus were significantly decreased by exposure to slough water; LTSO's for specimens of P. lilljeborgi were essentially the same in each lake water. P. lilljeborgi tolerates low oxygen much better than P. conventus but, in each case, death at 32 C is primarily caused by temperature. When the results of the experiments are shown graphically, the lines for P. conventus are steeper than those for any other aquatic invertebrate or vertebrate examined and suggest fundamental differences in the cause of death. In general, sphaeriids are "soft-water" species occupying lime-poor areas on the Precambrian Shield. The distribution and abundance of 13 species are related to depth but, in general, the relationships are not linear. Abundance is weakly-moderately correlated to maximum water temperature but because temperature is strongly correlated with depth the relationship of sphaeriids to this factor may also be non-linear. Sphaeriids are seldom found in those parts of lakes that regularly and periodically become devoid of oxygen. In general, abundance decreased as the level of total dissolved solids increased. Calcium is not limiting to sphaeriids in Saskatchewan lakes because they were abundant in lakes containing low calcium levels. Although correlations between average abundance and sulphate were negligible, this factor may be limiting to sphaeriids because they are not found in lakes containing more than about 2400 mg S04=/1. Factor analysis resolved the 17 environmental variables into four orthogonal factors: (1) water quality; (2) lake morphometri (3) depth, temperature, and oxygen; and (4) sodium, chloride, and sulphate. The importance of the 17 environmental variables and the four orthogonal factors to each species was assessed by step wise multiple linear regression analysis. The experimental and analytical phases of the study were complementary. Both phases indicated that intolerance to high temperature, low dissolved oxygen, and changes in water quality, are major factors in the ecology of P. aonventus. The analytical phase indicated morphometry is another important factor to this species. Both phases indicated these factors are not important in the ecology of P. Zittjeborgi but the analytical phase indicated that sodium, chloride, and sulphate ions are potentially important to this species.